Surveillance and monitoring systems have been utilized for years in assisted living facilities and in office buildings. In the future, use of such systems will expand to enable a variety of pervasive computing applications such as resource discovery, point-and-use interfaces, navigation, and augmented reality.
Systems that utilize infra-red technology such as infra-red transmitters and receivers have become common in recent years. In these systems, an infra-red transmitter delivers information such as an identity of the transmitter or a person carrying the transmitter to a infra-red sensor or a plurality of infra-red sensors within its range. No detailed spatial information is extrapolated from this information and thus only limited room-level resolution is provided by the systems. Recently, various improved indoor monitoring and tracking systems have been developed utilizing ultrasonic technology. These ultrasonic location systems perform trilateration, the principle of position finding using distance measurement. In these systems, short pulses of ultrasound are emitted from an ultrasonic transmitter. The times-of-flight of the pulse to ultrasonic receivers mounted at known locations are measured. Taking into account the speed of sound in air, the distance from the transmitter to each receiver is determined. Given three or more of such distance measurements, the three-dimensional position of the transmitter is calculated. The determination of a precise position of a transmitter or beacon is particularly more useful in monitoring and tracking applications. However, for a facility to replace an existing system utilizing infra-red technology with a new ultrasonic system would require significant cost and time which is undesirable.